The article I'm about to link to, by Oliver Reichenstein, is pretty terrible, but it's a good way for me to bring up something I've been meaning to talk about. First, the article: "Apple has been working on its file system and with iOS it had almost killed the concept of folders - before reintroducing them with a peculiar restriction: only one level! With Mountain Lion it brings its one folder level logic to OSX. What could be the reason for such a restrictive measure?" So, where does this crusade against directory structures (not file systems, as the article aggravatingly keeps stating) come from?

wallet inside a pocket, which is in turn inside another wallet
The restrictive nature of real world is limiting sometimes. But you can hide entire human with a wallet in a wallet if the latter is big enough.
A person who can't understand it is clearly suffer from serious mental disorder.

it's not hard to see how restricting it to one level make it easier for ordinary people to understand.

So who are these "ordinary" people who can't live in our hierarchical world?
Who just put a document in a folder on the shelf in a room in a house in a town in a country in a planet in a galaxy in the universe.
One level is clearly not enough.